1. Field of the Invention
The present invention relates in general to a method and an apparatus for pressing a sheet-like blank into a tubular or cylindrical container-like article. More particularly, the present invention is concerned with such pressing method and apparatus wherein a backward ironing step is effected on an intermediate workpiece prepared from the blank, such that the tubular portion of the workpiece is ironed in an axial direction from one axial end at which the tubular portion is open, toward the other axial end at which the tubular portion is closed by the bottom portion. The direction of ironing in the backward ironing step is opposite to that of the conventional forward ironing.
2. Discussion of the Related Art
Generally, a pressing operation to form a tubular container-like article from a sheet-like blank includes a step of drawing the blank into a tubular form having a tubular portion and a bottom portion which closes one of opposite axial ends of the tubular portion. The term "tubular" used herein is interpreted to mean cylindrical and other shapes such as polygons in transverse cross section of an intermediate workpiece or a final product in the form of a container, taken in a plane including the center line of the workpiece or product parallel to the axial or longitudinal direction.
Usually, the wall thickness of the tubular portion of the drawn article does not have a sufficiently high degree of uniformity in the axial direction. In some cases, therefore, the drawn article cannot be used as a final product or article of manufacture in the form of a tubular container, and is generally subjected to a further process step or steps such as an ironing operation performed on the tubular portion of the intermediate workpiece.
As shown in FIG. 20 by way of example, a conventional, widely known ironing process includes the steps of placing a cylindrical intermediate workpiece (blank) W on a columnar or cylindrical ironing punch 490 such that the leading end portion of the punch 490 is positioned within the cylindrical workpiece, and forcing the workpiece W and the punch 492 together into a hole of a die 302, in the axial direction with the bottom portion of the workpiece leading the punch 492, so that the cylindrical portion of the workpiece is ironed in the direction from the closed axial end toward the open axial end.
Commonly, the ironing operation indicated above follows the drawing operation, to obtain a sufficiently high degree of uniformity of the wall thickness of the tubular portion of the drawn workpiece, and improve the internal and external dimensions and shapes of the workpiece or article.
The assignee of the present invention developed a backward ironing process, and a device suitable for performing the backward ironing process, as disclosed in examined Japanese Utility Model Application published under Publication No 59-29770. This backward ironing device will be described by reference to FIG. 21, wherein the left half of the view shows an operating state of the device immediately after a backward ironing action is started, while the right half shows an operating state of the device immediately after the backward ironing action is terminated.
The backward ironing device is provided with a pushing punch 500 and a die 502. The pushing punch 500 is reciprocated in the longitudinal direction by a suitable drive device, the detailed discussion of which is not deemed necessary to understand the backward ironing device. The pushing punch 500 has a flat lower end face. The die 502 has a stepped die hole 504 formed therethrough, and is fixedly mounted on a base 508.
The die hole 504 has an upper small-diameter portion 510, and a lower large-diameter portion 512 having a larger diameter than the small-diameter portion 510. A flanged ironing punch 516 slidably engages the small-diameter portion 510 of the die hole 504, with a flanged sleeve 518 interposed therebetween. The ironing punch 516 is biased by a cushion pin 520, which is movable in the vertical direction. The ironing punch 516 and the sleeve 518 are normally held in their uppermost positions (indicated in the left half of the view of FIG. 21) under the biasing action of the cushion pin 520. In these uppermost positions, an outward flange 522 provided at the lower end of the sleeve 518 is in abutting contact with a shoulder surface of the die hole 504 between the small- and large-diameter portions 510, 512. The sleeve 518 has an axial length suitably determined in relation to the axial or height dimension of the ironed workpiece W. In the present example, the axial length of the sleeve 518 is determined such that the upper end of the sleeve 518 is located at an axially middle portion of the ironing punch 516. In operation, the cylindrical workpiece W is fitted on the upper portion of the punch 516 which is not surrounded by the sleeve 518. The upper portion of the punch cooperates with an ironing surface 523 of the small-diameter portion 512 of the die 502, to iron the cylindrical portion of the workpiece W in the axial direction from the open end toward the closed end, with the workpiece W and punch 516 being moved down relative to the die 502 by the pushing punch 500. The sleeve 518 functions to form the lower open end face of the cylindrical portion of the workpiece, such that the lower end face of the ironed cylindrical portion of the workpiece W is forced against the upper end face of the sleeve 518 immediately before the ironing action is terminated.
The ironing punch 516 and the cushion pin 520 are both hollow members, and an eject pin 524 extends through the bore in the cushion pin 520 and slidably engages the bore in the punch 516. The eject pin 524 is lowered with the workpiece W and punch 516 to the lowermost position (indicated in the right half of FIG. 21) at which the ironing action is terminated. Then, the eject pin 524 is moved up relative to the punch 516, to thereby push up the ironed workpiece W for removal from the punch 516.
There will be described in detail an operation of the backward ironing device of FIG. 21 to iron the workpiece W. The backward ironing operation consists of two major steps, namely, (1) a first step for positioning the workpiece W right above the ironing punch 516, by a gripping finger of a suitable work feed device, pushing the workpiece W on the upper portion of the punch 516 by the pushing punch 520, and forcing down the workpiece W and the punch 516 together into the die hole 504 to thereby iron the cylindrical portion of the workpiece W, and (2) a second step for moving up the pushing punch 520, ironing punch 516, sleeve 518 and workpiece W from the lowermost position (indicated in the right half of the view of FIG. 21), and separating the workpiece W from the ironing punch 516. The first step described above will be referred to as "backward ironing" or "backward ironing action" if appropriate.
Before the backward ironing operation is started, the pushing punch 500 is placed at its rest or non-operated position a given distance above the position indicated in the left half of the view of FIG. 21 at which the backward ironing action is started. In this rest position of the pushing punch 500, the workpiece W held by the gripping finger is positioned right above the upper end face of the ironing punch 516. Then, the pushing punch 500 is lowered from the rest position until the lower end face of the punch 500 comes into abutting contact with the outer surface of the bottom portion of the workpiece W. With a further downward movement of the pushing punch 500, the workpiece W is removed from the gripping finger and placed on the upper end portion of the ironing punch 516 such that the bottom portion of the workpiece W abuts on the upper end face of the ironing punch 516. The pushing punch 500 is further lowered to push down the workpiece W, ironing punch 516, sleeve 518, eject pin 524 and cushion pin 520, as a unit, against the biasing force of the cushion pin 520 acting on the ironing punch 516.
Thus, the workpiece W is lowered with its cylindrical portion being ironed by a cooperative action of the ironing punch 516 and the ironing surface 523 which partially defines the die hole 504. The backward ironing action is terminated when the lower end face of the ironing punch 516 abuts on the upper surface of the base 508. Namely, the base 508 serves as a stop which determines the lowermost position of the punch 516 and the workpiece W at which the backward ironing action is terminated. More precisely, the cylindrical portion of the workpiece W has a comparatively long constant-diameter section, and a comparatively short varying-diameter section which connects the constant-diameter section and the bottom portion of the workpiece W. The upper end of the constant-diameter portion is indicated at Pw in FIG. 22 which is an enlarged view of a part indicated at "A" in FIG. 21. On the other hand, the small-diameter portion 510 of the die hole 504 has a constant-diameter section which serves as the ironing surface 523, and an upper and a lower varying-diameter portions on the opposite sides of the constant-diameter portion. The upper end of the constant-diameter section or ironing surface 523 of the die hole 504 is indicated at Pd in FIG. 23 which is an enlarged view of a part indicated at "B" in FIG. 21. The backward ironing device is arranged so that the lower end face of the ironing punch 516 comes into abutting contact with the upper surface of the base 522 as indicated in the right half of FIG. 21, (1) when the lower end of the cylindrical portion of the workpiece W reaches or passes the lower end of the constant-diameter section (ironing surface 523) of the small-diameter portion 510 of the die hole 504, and (2) when the upper end (Pw) of the constant-diameter section of the cylindrical portion of the workpiece W reaches or passes the upper end (Pd) of the constant-diameter section of the small-diameter portion 510.
During the backward ironing action, the workpiece W is squeezed by the ironing punch 516, die 502 and pushing punch 500 such that the inner surface of the workpiece W is in pressing contact with the outer surface of the punch 516 while the outer surface of the workpiece W is in pressing contact with the ironing surface 523, lower end face of the punch 500, and the upper end face of the sleeve 518. Accordingly, substantially the entire areas of the inner and outer surfaces of the workpiece W are restricted under pressure by the punch 516 and the other members indicated above, so that the workpiece W is formed into a predetermined shape with high accuracy. This ironing action involves a flow of the material of the workpiece W as a result of reduction in the wall thickness of the cylindrical portion, in the axial direction from the open end toward the closed end (bottom portion), and a surplus amount of stock of the material fills a space left defined by the lower end face of the punch 500, the ironing surface 523 and the original outer arcuate contour of the varying-diameter section between the constant-diameter section and the bottom portion of the workpiece W, as indicated in FIG. 23.
Upon completion of a backward ironing pass with the ironing punch 516 abutting on the base 508, the pushing punch 500 is raised, permitting the workpiece W and the ironing punch 516 to be pushed up together by the cushion pin 520, from the lowermost position at right in FIG. 21 to the uppermost position at left in the same figure. The pushing punch 500 is further raised to its rest or non-operated position, while the eject pin 524 is moved up relative to the ironing punch 516, until the upper end of the eject pin 524 is located some distance above the upper end of the punch 516, whereby the workpiece W is removed from the punch 516. The thus ironed workpiece W is then clamped by the gripping finger of the work feed device, and transferred to a next station in the production line in question.
In the conventional forward ironing operation in which the cylindrical portion of the workpiece W is ironed in the axial direction from the closed end (bottom portion) to the open end, as illustrated in FIG. 20, the cylindrical portion of the workpiece W is subject to a compressive stress arises in the circumferential direction, and to a tensile stress in the axial direction. In the backward ironing operation as generally illustrated in FIG. 24, on the other hand, the ironing action proceeds in the axial direction from the open end toward the closed end, with a movement of a pushing punch 500' to force the workpiece W and an ironing punch 516' into a die hole in a die 502'. During the backward ironing operation, compressive stresses arise in the cylindrical portion of the workpiece W, in both the circumferential direction and the axial direction. In other words, only the compressive residual stresses remain within the cylindrical portion of the workpiece W, without a room for a tensile stress arising in the workpiece.
When the conventional forward ironing operation is applied to a workpiece or blank made of a stainless material such as an austenite stainless steel having an unstable austenite phase or a high-strength material such as a high-tensile-strength steel, tensile stresses tend to remain as internal or residual stresses in the cylindrical portion (adjacent the open end, in particular) of the ironed workpiece, and the workpiece tends to relatively easily suffer from aging crack (season crack or delayed crack) in the axial direction beginning at its open end, without external forces acting thereon, when the workpiece is left in the atmosphere for a short time (several minutes to several days). An example of the workpiece suffering from such aging crack is shown in FIG. 25. If the forward ironing operation is applied to a workpiece of an ordinary metal material such as carbon steel, tensile stresses tend to remain in the cylindrical portion (adjacent to the open end in particular) of the workpiece, and the workpiece is likely to undergo strain hardening with a result of increase in the brittleness. In this case, the workpiece easily cracks in the axial direction beginning at its open end of the cylindrical portion.
If the workpiece is subjected to the backward ironing operation in place of the forward ironing operation, on the other hand, compressive stresses necessarily remain in the cylindrical portion (at least at its open end section) of the workpiece, irrespective of the material (stainless steel having an unstable austenite phase, high-strength material, or ordinary metal material), and the ironed workpiece is relatively free from the cracking experienced in the conventional forward ironing operation.
The assignee of the present invention proposed a pressing process as disclosed in the above-identified Publication No. 59-29770, in which the workpiece or blank is subjected first to a drawing operation and then to a backward ironing operation as explained above.
However, the following drawback was found in the proposed pressing process including the drawing and backward ironing operations which are performed in this order.
For improving the uniformity of the wall thickness of the tubular portion of the intermediate workpiece drawn, it is necessary to iron the tubular portion with a considerably high ironing ratio or percent (wall thickness reduction ratio of the ironed workpiece with respect to the thickness before ironing, i.e., thickness of the drawn workpiece). It was found in the case of the backward ironing, however, that the higher the ironing ratio, the higher a possibility of a space being formed at an arcuate inner fillet (inner corner surface) indicated at 528 in FIG. 22 between the bottom and cylindrical portions of the ironed workpiece W, more specifically, between the surface of the fillet corner 528 and the facing surface of the ironing punch 516, as shown in FIG. 23. The formation of such a space (so-called "piping defect") along the inner fillet 528 appears to arise from a material flow of the workpiece W from the constant-diameter section to the varying-diameter section between the constant-diameter section and the bottom portion, whereby the varying-diameter section tends to buckle outwardly at an arcuate outer round (outer corner surface) indicated at 526 in FIG. 22, which corresponds to the inner fillet 528. This buckling causes a space to be formed between the inner fillet 528 and the corresponding corner of the punch 516. Therefore, there is a limitation in the ironing ratio or percent in the backward ironing operation, and the backward ironing operation is not satisfactory for even or uniform wall thickness of the ironed workpiece.
While the backward ironing process is substantially free of cracking of the ironed workpiece as described above, the backward ironing process as performed by the device of FIG. 21 has the following problem.
That is, where there exists a relatively narrow space between the outer round 526 of the workpiece W and the lower end of the pushing punch 500, the formation of a space ("piping defect") along the inner fillet 528 of the workpiece W and the ironing punch 516 is less likely to occur. If the space between the lower end of the punch 500 and the outer round 526 is relatively ample as in the case of FIG. 22, the surplus amount of stock of the workpiece material can be sufficiently accommodated in that ample space. This advantage, however, is provided at an expense of an increased space along the inner fillet 528, which space may easily grow into a defect as indicated at 530 in FIG. 23. This defect 530 is caused by movements of mutually facing masses of the material toward each other at the inner corner 528 of the workpiece W, so as to fill a substantially entire portion of the space originally formed along the inner corner 528.
The above defect 530 is likely to take place if the space between the outer round or corner surface 526 and the end face of the pushing punch 500 is comparatively large, irrespective of the ironing ratio. A considered reason for this phenomenon is that the outer corner surface 526 of the workpiece W is not restricted by the punch 200 and the die 502, and is relatively easily permitted to buckle or bend outwardly of the punch 516, as the material flows from the cylindrical portion toward the bottom portion of the workpiece W in the process of the backward ironing in the same direction as that of the material flow. The buckling at the outer corner surface 526 involves the formation of an inner space along the inner fillet or corner surface 528. Thus, the inner surface of the ironed workpiece W does not accurately follow the profile of the ironing punch 516.